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Talc is widely distributed over the Earth's surface and is predicted to be formed in various tectonic settings. Talc is a very soft and anisotropic sheet silicate showing very low friction behavior. Therefore, the formation of talc is expected to weaken the strength of talc-bearing rocks and may be associated with the initiation of subduction, and with a decrease in the coupling coefficient resulting in aseismic movements along faults and shear zones within subduction zones. For these reasons, understanding the crystallographic preferred orientation (CPO) of talc is important to quantify the anisotropy and physical properties of the host rock. However, it is difficult to measure a significant number of talc crystal orientations and to evaluate the accuracy of the measurements using electron-backscattered diffraction (EBSD). Therefore, talc CPO has not been reported, and there is uncertainty regarding the estimation of the strength of deformed talc-bearing rocks. Using methods developed for antigorite, we report the first successful EBSD measurements of talc CPO from a talc schist formed due to Simetasomatism of ultramafic rocks by subduction zone fluids. We used a combination of W-SEM and FE-SEM measurements to examine domains of various grain sizes of talc. In addition, we used TEM measurements to evaluate the accuracy of the EBSD measurements and discuss the results of talc CPO analysis. Talc CPO in the present study shows a strong concentration of the pole to the (001) plane normal to the foliation. The strongest concentration of the [100] direction is parallel to the lineation. The talc schist produces similar S-wave splitting and P- and S-wave anisotropy as antigorite schist in deeper domains, thus identifying talc-rich layers in subduction zones may require a combination of geophysical surveys, seismic observations, and anisotropy modeling. The presence of strong talc CPO in rocks comprising the slab–mantle interface boundary may promote spatial expansion of the slip area during earthquakes along the base of the mantle wedge.

Silica activity in fluids is a key factor that controls reaction pathways during the hydrothermal alteration of olivine in the oceanic lithosphere. In this study, we conducted hydrothermal experiments (300 °C, 8.58 MPa) on the olivine (Ol)–quartz (Qtz)–H2O system to understand the coupling between silica transport and olivine alteration. Mineral powders were reacted with 0.5 mol kg−1 NaCl solution in a tube-in-tube type vessel, and the spatial distribution of reactant and product minerals was investigated after the experiments. Alteration zones formed in the Ol-hosted region after 2055 hours of reaction. With increasing distance from the Ol–Qtz boundary these were: talc; talc + serpentine; and serpentine + magnetite + brucite. Talc formed 0–2.3 mm from the Ol–Qtz boundary in the Ol-hosted region, and brucite formed >5 mm from the Ol–Qtz boundary in the Ol-hosted region. No secondary minerals formed in the Qtz-hosted region. The observed mineral distribution was modeled using a reactive transport model that simulated the coupling between SiO2(aq) diffusion and seven silica-controlling reactions. An inverse modeling framework, which combines a reactive transport model with an exchange Monte Carlo method, was used to parameterize the diffusivity of SiO2(aq) and the rate constants of the seven overall reactions. Our model shows that the rate of hydration in the serpentine + metastable talc zone with intermediate silica activity was higher than in the serpentine, serpentine + brucite, and talc zones, suggesting that the silica activity of the reacting fluid has a significant control on the rate of hydrothermal alteration of mantle peridotite by crustal fluids. Moreover, the model suggests that the rate-control process changed from being surface- to transport-controlled over the course of the experiments. We suggest that dynamic changes in rate control process are important contributors to the formation of metasomatic zoning and heterogeneous hydration patterns within the oceanic lithosphere.

Heterogeneous reactions are chemical reactions that occur at the interfaces of multiple phases, and often show a nonlinear dynamical behavior due to the effect of the time-variant surface area with complex reaction mechanisms. It is important to specify the kinetics of heterogeneous reactions in order to elucidate the microscopic elementary processes and predict the macroscopic future evolution of the system. In this study, we propose a data-driven method based on a sparse modeling algorithm and sequential Monte Carlo algorithm for simultaneously extracting substantial reaction terms and surface models from a number of candidates by using partial observation data. We introduce a sparse modeling approach with non-uniform sparsity levels in order to accurately estimate rate constants, and the sequential Monte Carlo algorithm is employed to estimate time courses of multi-dimensional hidden variables. The results estimated using the proposed method show that the rate constants of dissolution and precipitation reactions that are typical examples of surface heterogeneous reactions, necessary surface models, and reaction terms underlying observable data were successfully estimated from only observable temporal changes in the concentration of the dissolved intermediate products.

Metaharzburgite and metadunite in the ultramafic body of the Naran Massif in the Khantaishir Ophiolite, western Mongolia, record multi-stage processes of serpentinization (antigorite, lizardite + brucite, then chrysotile). Bulk-rock chemistry and the compositions of primary olivine (P-olivine) and Cr-spinel suggest that the alteration occurred in the forearc mantle. In the metaharzburgite, a novel occurrence of fine-grained (10–50 μm) secondary olivine (S-olivine) takes the form of aggregates (a few millimeters across) with bands of antigorite. The S-olivine has higher Mg# values (0.96–0.98) than the P-olivine (Mg# = 0.92–0.94) and contains inclusions of clinopyroxene and magnetite. The P-olivine has been replaced by antigorite and magnetite. Mesh textures of lizardite + brucite are developed in both P- and S-olivine. The microtextures and chemical compositions of minerals suggest that S-olivine aggregates were formed by pseudomorphic replacement of orthopyroxene related to multi-stage hydration processes. Assuming the mantle wedge conditions beneath a thin crust, orthopyroxene was first replaced by S-olivine + talc at high temperatures (500–650 °C at ~ 0.5 GPa). With cooling to ca. 400–500 °C and fluid supply, talc transformed to antigorite with the release of silica. During this stage, P-olivine was also transformed to antigorite by consumption of silica released from orthopyroxene decomposition. At temperatures below 300 °C, lizardite + brucite ± magnetite formed from the remaining P- and S-olivine grains. The formation of S-olivine presented in this study contrasts with the commonly ascribed process of deserpentinization. Taking into account the geochemical data for the studied ultramafic rocks and those previously reported for mafic rocks, our results suggest that mantle wedge beneath thin crust was hydrated in response to continuous cooling and fluid supply from a subducting slab after subduction initiation.

Mineral–fluid equilibria in supercritical geothermal reservoirs is poorly understood, due to the non-applicability of the Helgeson–Kirkham–Flowers (HKF) electrostatic model for aqueous species in low-density regions. We experimentally investigated the dissolution equilibria of albite–K-feldspar–quartz in hydrothermal solutions at 400, 420 °C and 20–35 MPa, and compared the experimental results with thermodynamic calculations. The thermodynamics data based on the HKF model available through SUPCRT92 were extrapolated into the low-density regions with water density. Our results suggest that the extrapolated data are useful as a first approximation for evaluating feldspars–fluid equilibria under supercritical and low-density hydrothermal conditions.

Surface heterogeneous reactions are chemical reactions with conjugation of multiple phases, and they have the intrinsic nonlinearity of their dynamics caused by the effect of surface-area between different phases. We propose a sparse modeling approach for extracting nonlinear dynamics of surface heterogeneous reactions from noisy observable data. We employ sparse modeling algorithm and sequential Monte Carlo algorithm to partial observation problem, in order to simultaneously extract substantial reaction terms and surface models from a number of candidates. Using our proposed method, we show that the rate constants of dissolution and precipitation reactions, which are typical examples of surface heterogeneous reactions, necessary surface models and reaction terms underlying observable data were successfully estimated only from the observable temporal changes in the concentration of the dissolved intermediate product.

Water&ndash, rock interaction in surface and subsurface environments occurs in complex multicomponent systems and involves several reactions, including element transfer. Such kinetic information is obtained by fitting a forward model into the temporal evolution of solution chemistry or the spatial pattern recorded in the rock samples, although geochemical and petrological data are essentially sparse and noisy. Therefore, the optimization of kinetic parameters sometimes fails to converge toward the global minimum due to being trapped in a local minimum. In this study, we simultaneously present a novel framework to estimate multiple reaction-rate constants and the diffusivity of aqueous species from the mineral distribution pattern in a rock by using the reactive transport model coupled with the exchange Monte Carlo method. Our approach can estimate both the maximum likelihood and error of each parameter. We applied the method to the synthetic data, which were produced using a model for silica metasomatism and hydration in the olivine&ndash, quartz&ndash, H2O system. We tested the robustness and accuracy of our method over a wide range of noise intensities. This methodology can be widely applied to kinetic analyses of various kinds of water&ndash, rock interactions.

Isocon analysis has been widely applied to various geoscientific problems as a simple standard tool for quantitative estimation of material transfer. Despite its usefulness, similar to all material transfer calculations, this method generally requires the presumptive specification of immobile elements or the assumption of conservation of mass or volume. However, the validity of such assumptions is particularly controversial. Here we propose a novel data-driven method that automatically estimates the mass gain or loss of elements based on compositional data of multiple samples that have been altered from the original rock without assuming immobile elements. The proposed method uses a mathematical framework, called sparse modeling, that can extract essential information from high-dimensional datasets based on the sparsity of the system. In this case, it is assumed that some elements show higher immobility than others (i.e., the material transfer of such elements is near zero). By optimizing the evaluation function, the immobile elements are automatically selected. By inputting only the bulk compositional datasets, the user can obtain the material gain or loss with total mass change ratio for each sample relative to the reference (original) rock. The effectiveness of the method is validated and discussed using synthetic and natural sample data. Software packages are available from the authors in MATLAB function and Excel workbook forms.